The variable rate application of dry fertilizer typically involves a conveyor that moves fertilizer through a fertilizer box to a dispense opening. Such systems may determine a dispense rate based on the conveyor rate and the size of the dispense opening. This calculation is typically based on the assumption that the effective dispense opening remains constant regardless of the speed of the conveyor, i.e., that amount of fertilizer dispensed for a given dispense opening and length of conveyor run is generally constant, regardless of conveyor speed. For example, the dispensing system may be calibrated using the amount of fertilizer dispensed during a single revolution of a conveyor at a predetermined dispense opening size when the conveyor is run at a baseline speed and this value is then used for determining dispense rates at other conveyor speeds. At this speed the effective dispense opening is approximately the size of the actual dispense opening.
While the afore-mentioned method works fairly well when the conveyor is operated at a speed near that of the baseline speed, it is problematic when the conveyor is run at much higher speeds, at which the fertilizer particles behave differently as they attempt pass through the dispense opening. For example, the increased speed of the conveyor causes increased friction and the fertilizer particles tend to collide together and compact as the fertilizer attempts to move through the dispense opening. This results in a dispense opening having a smaller effective size when the conveyor is run at higher speeds, thereby disrupting the linear relationship of an increase in speed and amount dispensed that might be expected. This problem is particularly acute when increasing the speed of the conveyor quickly by a large amount, which may occur with precision farming systems. This smaller effective dispense opening results in less fertilizer being dispensed per conveyor revolution for a given dispense opening actual (physical), thereby resulting in inaccurate dispense rates. In an effort to overcome this problem, previous systems have tried to automate change the physical size of the dispense opening itself by employing an automated movable gate that covers the dispense opening in an effort to manipulate the size of the dispense opening to obtain a specified effective dispense rate. These systems have proven to be costly and unreliable.